1. Field of Invention
A coaxial diffuser optical fiber wave guide using a diffuser along the fibers axis to effect an internal scatter of incident radiation to permit a fraction of the scattered incident radiation to enter a guided mode in the optical fiber wave guide and be propagated to the fiber end thereby creating a concentration of the incident radiation energy at the fiber end.
2. Description of the Prior Art
Certain radiation energy can be utilized more efficiently when concentrated, for example solar energy radiation or luminescence from photophosphorescence. The phenomenon of total reflection at the interface between material of different refractive indexes, known as Snell's law, has been used to concentrate radiation energy. This principle is the basis of dielectric wave guides such as optical fibers. However, with respect to using dielectric wave guides for concentration incident radiation, an important consequence of Snell's law is that a source of radiation outside the wave guide core cannot contribute to guided modes. The incident radiation will reflect and refract at the wave guide boundaries, but none of the wave's energy will be trapped as a guided wave.
One method to overcome this problem is used by Zewail (U.S. Pat. No. 4,227,939). Fluorescent dye in the wave guide core absorbs the incident radiation and by fluorescence emission scatter the energy within the core to permit photon capture in guided modes. The method requires a variety of fluorescent material to cover the solar spectrum due to the specific absorption wave lengths associated with each fluorescent dye.
The complication of this approach arises from the need to maximuze the absorption of incident photons without creating unacceptable transmission losses in the guided modes of the wave guide. The device is a slab type dielectric wave guide in which the trapped photons are reflected repeatedly across the plate's central axis and through various fluorescent absorbers. A fraction of the photons energy is dissipated during each fluorescent cascade event which causes a lossy collector.
Commercially available optical fibers can only capture incident radiation in guided modes through events cuased by imperfections in the fibers. Although these fibers are satisfactory in applications such as cores for short luminescent fiber markers, the inability of the incident radiation, in this example backscatter photophosphorescence, to enter guided modes limits the fibers ability to concentrate the incident radiation.